What can I tell about the condition of a wine by looking at the cork?
Not much. It's sort of like judging a car based on the condition of the tires. I've had splendid old bottles topped with corks that shredded on contact. (Coffee filters work wonders in such cases.) Even mould – so long as it's on the cork's top side, nearest the tip of the bottle – tends to signify nothing.
Waiters make a show of placing the cork in front of you on the table for another reason entirely. The ritual became popular long ago when vintage fraud was on the rise. Restaurant patrons got wise to the fact that many establishments had been serving up fakes, uncorked out of sight and brought to the table already open. To battle counterfeiters, châteaux began branding their corks and even stencilling harvest years on them. So, the onus was on restaurants to uncork the wine at the table and display its authenticity using the cork.
In other words, you're supposed to examine the cork with a quick glance. Don't bother sniffing it; you'll learn nothing. You can't even tell if a wine is "corked" – that is, infected with a contaminant called TCA, which smells like mouldy cardboard. TCA can only be discerned by smelling or tasting the wine, not the cork.
The Flavour Principle by Lucy Waverman and Beppi Crosariol (HarperCollins) won top prize for best general English cookbook at the 2014 Taste Canada Food Writing Awards.